1. Field of the Invention
The present invention relates to a spread illuminating apparatus as a lighting means for a liquid crystal display apparatus, and more specifically to a side light type illuminating apparatus using a point light source.
2. Description of the Related Art
Recently, a liquid crystal display (hereinafter referred to as “LCD” as appropriate) apparatus is extensively used as a display device for electronic equipments, such as personal computers, cellular phones, and the like. Since the liquid crystal, unlike a cathode ray tube, does not emit light by itself, a transmission type LCD apparatus requires a lighting means for illuminating its liquid crystal panel, and also a semi-transmission type LCD apparatus which utilizes outside light must have an auxiliary lighting means for enabling a usage in the dark. A side light type illuminating apparatus mainly comprises a light conductive plate and a light source disposed at a side surface of the light conductive plate, readily enables its profile to be lowered, and therefore can be suitably used as a lighting means for the LCD apparatus, for which a cold cathode tube, that is a line light source, is conventionally used.
Conventionally, in such a spread illuminating apparatus, outgoing light is converged within the viewing angle of a liquid crystal panel, or the like as an object to be illuminated, thereby enhancing brightness within the viewing angle (front brightness), which is practically important (refer to, for example, Japanese Patent Application Laid-Open No. H10-253960). FIG. 12 is an exploded perspective view of a conventional side light type spread illuminating apparatus 100 having such a light converging means as described above. The spread illuminating apparatus 100 comprises a light conductive plate 101 defining a light entrance surface 101c, a light reflection surface 101b and a light exit surface 101a, a line light source 102 disposed at the light entrance surface 101c of the light conductive plate 101, and a prism sheet 103 disposed on the light exit surface 101a of the light conductive plate 101.
The light conductive plate 101 is formed of a transparent resin material, such as methacrylate resin, polycarbonate resin, and the like. On the light exit surface 101a which is one major surface of the light conductive plate 101, a plurality of triangular prisms are formed which are shaped identical with one another, extend in a direction orthogonal to the light entrance surface 101c , and which are arrayed at a regular interval. On the light reflection surface 101b which is the other major surface of the light conductive plate 101 opposite to the light exit surface 101a, a diffuse or regular reflecting means (not shown) is provided which reflects light toward the light exit surface 101a so that the light impinges on the light exit surface 101a at an angle less than an critical angle. The prism sheet 103 is formed of a transparent resin material, for example a PET film, and has, on a major surface (prism surface) 103a thereof, a plurality of triangular prisms which are formed of a transparent resin material, such as methacrylate resin, polycarbonate resin, and the like, and which extend in one same direction. The prism sheet 103 is disposed on the light conductive plate 101 such that the prism surface 103a faces the light exit surface 101a with their respective triangular prisms extending orthogonally to each other.
In the following discussion, a direction orthogonal to the light entrance surface 101c of the light conductive plate 101 is defined as an X-axis direction, a direction normal to the light exit surface 101a of the light conductive plate 101 is defined as a Z-axis direction, and a direction perpendicular to both the X-axis and Z-axis directions is defined as a Y-axis direction. Also, planes defined by the coordinate axes (X, Y, Z) shown in FIG. 12 are referred to as an XY plane, a YZ plane, and an XZ plane, which will be used also for any planes parallel thereto.
In the spread illuminating apparatus 100, light emitted from the line light source 102 is introduced into the light conductive plate 101 through the light entrance surface 101c thereof, then partly exits out from the light exit surface 101a while traveling inside the light conductive plate 101 with repeated reflections at the light reflection surface 101b and the light exit surface 101a, and finally illuminates a liquid crystal panel, or the like as an object illuminated. When the light exits out from the light exit surface 101a, the YZ plane directivity of the light is compensated with respect to the XZ plane by means of the triangular prisms formed on the light exit surface 101a, and then the XZ plane directivity of the light is compensated with respect to the YZ plane by means of the triangular prisms formed on the prism surface 103a of the prism sheet 103, whereby brightness for the Z-axis direction neighborhood, which is the front brightness, is enhanced. This arrangement is advantageous compared with an arrangement in which two prism sheets are disposed on a light exit surface of a light conductive plate with their respective prisms extending orthogonally to each other such that their respective plain surfaces opposite to the prism surfaces face toward the light exit surface of the light conductive plate, in that the number of expensive prism sheets is reduced, and in that light exiting out the light conductive plate is not reflected at the plane surfaces of the prism sheets so as to go back to the light conductive plate. It is known that if the light reflection surface 101b is provided with a regular reflecting means comprising, for example, V-shaped grooves extending in the Y-axis direction so that light reflected by such a reflecting means falls incident on the light exit surface 101a at an angle slightly smaller than a critical angle, then the light exits out from the light exit surface 101a with a narrow directivity inclined in the X-axis direction within the ZX plane, whereby the light converging efficiency is improved resulting in further enhanced brightness.
Recently, a white light emitting diode (LED) with enhanced performance is increasingly used as a point light source in a spread illuminating apparatus for the purpose of downsizing and lowering power consumption. However, while a line light source emits light with comparatively uniform brightness along its length direction, a plurality of point light sources, which are disposed in a line and parallel to a light entrance surface of a light conductive plate, have respective different brightness distributions with their peaks positioned at the front direction, which causes light outgoing from the apparatus to involve brightness variation unless a uniformizing means is provided. For example, in the spread illuminating apparatus 100 of FIG. 12, if a plurality of point light sources, in place of the line light source 102, are disposed parallel to the light entrance surface 101c of the light conductive plate 101, when light exits out from the light emit surface 101a, the brightness distribution of the outgoing light has its non-uniformity diminished to some degree with respect to the Y-axis direction by means of the prisms formed on the light emit surface 101a, but each point light source normally has emission lines at its both sides in a symmetrical manner thus failing to lead to resolving the brightness variation problem. Especially, when the light reflecting surface 101b is provided with the regular reflecting means described above, the non-uniformity of the brightness distribution of light emitted from the light source is materially reflected thus making the aforementioned problem further noticeable.
To address the problem, the present inventor proposed previously in Japanese Patent Application No. 2002-368805 a spread illuminating apparatus which incorporates a light diffusing means. According to the first embodiment of the aforementioned Japanese Patent Application No. 2002-368805, the light diffusing means disclosed therein comprises a plurality of prisms of various kinds extending in the direction orthogonal to a light entrance surface of a light conductive plate. The prisms include circular arcs in cross-section taken along a line perpendicular to the extension direction, and the maximum tangential angles of respective circular arcs with respect to a principal virtual plane are different from one another. By the light diffusing means thus structured, light traveling in the light conductive plate is caused to be diffused in the direction parallel to the light entrance surface, which enables a spread illuminating apparatus with point light sources to brightly and uniformly illuminate an object.
However, the prisms, which constitute the light diffusing means of the light conductive plate in the spread illuminating apparatus disclosed in the aforementioned Japanese Patent Application No. 2002-368805, may have their respective heights differing from one another, and therefore a prism sheet to be disposed on the light conductive plate is supported only partly by some of the prisms of the light diffusing means that have relatively large heights, rather than entirely by all of the prisms. Consequently, when the illuminating apparatus is subjected to vibration or change of ambient temperature, the contact portions of both the light conductive plate and the prism sheet may have scratches, and the scratches are viewed as emission lines (defects) thus degrading the display quality on the object illuminated.